1. The Field of the Invention
The invention generally relates to improving communications in an optical fiber. More specifically, the invention relates to counteracting the effects of echoes caused by discontinuities in fiber-optic networks.
2. Description of the Related Art
In the field of data transmission, one method of efficiently transporting data is through the use of fiber-optics. Digital data is propagated through an optical fiber using light emitting diodes or lasers. Light signals allow for high transmission rates and high bandwidth capabilities. Also, light signals are resistant to electro-magnetic interferences that would otherwise interfere with electrical signals. Optical fibers do not typically allow portions of the light signal to escape from the optical fiber as can occur with electrical signals in wire-based systems.
In a typical fiber-optic network, the transmission and reception of data is not strictly limited to optical signals. Digital devices such as computers may communicate using both electronic and optical signals. As a result, optical signals need to be converted to electronic signals and electrical signals need to be converted to optical signals. To convert electronic signals to optical signals for transmission on an optical fiber, a transmitting optical subassembly (TOSA) is often used. A TOSA uses a electronic signal to drive a laser diode or light emitting diode to generate an optical signal. When optical signals are converted to electronic signals, a receiving optical subassembly (ROSA) is used. The ROSA has a photo diode that, in conjunction with other circuitry, converts the optical signals to electronic signals.
Because most computers and other digital devices both transmit and receive signals, most computers need both a TOSA and a ROSA to communicate through optical fibers. A TOSA and ROSA can be combined into an assembly generally referred to as a transceiver. Accordingly, most computers in a fiber-optic network are configured to communicate in a bidirectional nature, meaning that they can both transmit and receive signals on the fiber-optic network.
One method of achieving bidirectional communication is through the use of two optical fibers. The first fiber can be used to transmit optical signals and the second fiber can be used to receive optical signals. It is often desirable to limit the number of optical fibers between two communication points to save on material costs and installation. The number of fibers in an optical network can be limited by both sending and receiving signals on the same optical fiber, which is possible because of the directional nature of an optical signal that is propagated along an optical fiber.
Various challenges arise when transmitting signals in both directions along a single optical fiber. For example, while generally bidirectional fiber-optic communication is achievable because of the directional nature of the fiber-optic signals, a transmitted signal that is reflected by some discontinuities or irregularities in the fiber-optic network can results in a portion of a signal being reflected. This reflected signal might then be interpreted by the transceiver that sent the signal as a portion of a received signal.
Reflected signals add noise to the signal that is received by the transceiver. Additional noise in the received signal reduces the signal to noise ratio (SNR) which is a measure of signal quality. The SNR is used to calculate the bit error rate (BER), which is a rate at which errors occur when signals are interpreted. In one exemplary fiber-optic communication standard, the 10 Gigabit Ethernet standard, the BER that is within the acceptable limits defined by the standard is 10−12, or about 1 error in 1,000,000,000,000 bits. To any extent that reflected signals cause a BER greater than that allowed by the standard in which the signal operates, those reflected signals should be mitigated such that the total BER falls within the acceptable limits defined by the standard.
One method of dealing with these reflected signals is to use echo cancellation. Echo cancellation essentially counteracts, at the transceiver, any reflected signal with a canceling signal such as one that is equal but opposite in magnitude to the reflected signal. Echo cancellation has not been widely used because it is expensive in terms of computing resources to implement. For example, to generate the canceling signal, information should be known about discontinuities on the fiber-optic network in terms of what sorts of reflections these discontinuities cause. Namely, to cause the echo cancellation signal to be generated at the appropriate time, the designer of an echo cancellation configuration should know the amount of time for a signal to reach the discontinuity, to be reflected by the discontinuity and to return to the transceiver. Further, for the echo cancellation signals to be of the proper magnitude, the designer of an echo cancellation configuration should know what percentage of a signal will be reflected back to the transceiver. The data transmitted on the network is used to create the canceling signal for data that is reflected. Thus, any data transmitted on the network should be maintained or remembered for an amount of time equal to the time for the signal to travel to the discontinuity, to be reflected by the discontinuity and to return to the transceiver. Further, information may have to be stored regarding any discontinuities that exist throughout the entire fiber-optic network that causes signals to be reflected to the transceiver.
One conventional method of bidirectional communication along a single optical fiber that addresses reflection challenges involves the use of lasers with different wavelengths. In a bidirectional communication configuration, one laser wavelength (e.g. 1550 nm) is used to transmit signals in one direction, while a second wavelength (e.g. 1310 nm) is used to transmit signals in the opposite direction. Thus, any reflected signals can be filtered out by an appropriate band-pass filter that is configured to only allow a certain band of frequencies to pass through or a notch filter that is configured to allow a very narrow band of frequencies to pass through. A receiver configured to detect transmissions from a 1310 nm laser, for example, filters out reflections at 1550 nm, and vice versa.
One drawback with this configuration is that it requires two types of transceivers that are complementary with different transceivers being used at the two communications devices that are engaging in the bidirectional communication. For example, one of the two communications devices should have a transceiver with a 1550 nm transmitter and a 1310 nm receiver. In contrast, the other of the two communications devices should have a complementary transceiver having a 1310 nanometer transmitter and a 1550 nanometer receiver. Requiring two types of transceivers increases production and maintenance costs. In addition, these types of receivers can lead to communication problems due to incompatibility. In other words, a device with a 1310 nm receiver can only receive data from devices that transmit data using a 1310 nm laser.